1. Technical Field
The subject matter described herein relates to a thermotropic optical shutter device that incorporates one or more coatable polarizers. Implementations of such devices have application in passive or active light-regulating and temperature-regulating films, materials and devices, including construction materials.
2. Description of the Related Art
The problem of controlling the flow of radiant energy, e.g., light and heat, in particular in applications such as regulating solar heat gain in buildings and in other applications has previously been addressed using many optical and infrared methodologies. Photodarkening materials have been used for decades, for example, in sunglass lenses, to selectively attenuate incoming light when stimulated by ultraviolet (UV) radiation. When incorporated into windows, such materials can be used to regulate the internal temperature of a structure by darkening to attenuate bright sunlight, and by becoming transparent again to allow artificial light or diffuse daylight to pass through unimpeded. Such systems are passive and self-regulating, requiring no external signal other than ambient UV light in order to operate. However, because they are controlled by UV light rather than by temperature, such systems are of limited utility in temperature-regulating applications. For example, they may block wanted sunlight in cold weather as well as unwanted sunlight in hot weather. They also may not function if placed behind a UV-blocking material such as the transparent, spectrally-selective and low-emissivity coatings that are commonly employed in the window industry.
U.S. Pat. No. 7,755,829 to Powers et al. discloses an optical filter composed of a thermotropic, low clearing point, twisted nematic liquid crystal sandwiched between two reflective polarizers that can be used as a window film or other light- and heat-regulating building material. Similarly, in U.S. Pat. No. 8,169,685 to Powers et al., a thermodarkening filter composed of a low clearing point liquid crystal sandwiched between two absorptive polarizers is disclosed as a component of building materials, e.g., as a window film. In addition, U.S. Patent Application Publication No. 2009/0268273 to Powers et al. discloses a thermotropic optical filter incorporating both absorptive and reflective polarizers and U.S. Patent Application Publication Nos. 2010/0045924 and 2010/0259698 to Powers et al. disclose thermotropic, light-regulating liquid crystal devices that do not require polarizing substrates at all.
There are also numerous types of linear polarizers, including absorptive, diffusive, and reflective types made from stretched polymers. There are further linear, reflective wire grid polarizers, which are less commonly used but are nevertheless familiar structures. Finally, circular polarizers made from a coatable film of cholesteric liquid crystals, or CLCs, are also known. Thermotropic devices incorporating all of these polarizer types have been disclosed in U.S. Pat. No. 7,755,829 and related patents and patent applications to Powers and McCarthy.
Coatable linear polarizers are described, for example, in a scientific paper entitled “A novel thin film polarizer from photocurable non-aqueous lyotropicchromonic liquid crystal solutions” (Yun-Ju Bae, Hye-Jin Yang, Seung-Han Shin, Kwang-Un Jeong and Myong-Hoon Lee, J. Mater. Chem., 2011, 21, 2074). Korean researchers Bae et al. disclose a composition of matter which, when shear-coated and UV cured onto a glass surface, forms a thin-film polarizer. Shear may be induced by a number of different coating processes, including doctor blade coating, Mayer rod coating, roll coating, and gravure coating. Such processes are well described including, for example, in U.S. Patent 2002/0160296 to Wolk et al.
These shear-coated linear polarizers typically consist of lyotropic, chromonic liquid crystals (LCLCs), which are essentially dye molecules that have been functionalized so they behave as liquid crystals. These materials may be prepared using common synthetic organic chemistry techniques. In the base solution disclosed in Bae et al., the LCLC was mixed with a prepolymer material and then cured to form a polymer matrix with LCLC interspersed, providing increased mechanical stability to the system. These coatings are typically applied to either glass or thin film polymer substrates. Coatable polarizers made from chromonic liquid crystal polymers are also known.
Polymer-stabilized liquid crystal formulations and guest-host liquid crystal formulations may have both chemical and physical similarities to chromonic liquid crystal films, as both may be curable liquids that form highly ordered optical materials. It is also possible to incorporate polymerizable groups such as vinyl, acrylate, epoxide, isocyanate, etc. directly onto the LCLC itself to produce an integrated system containing a polymerizable LCLC. This strategy has proven effective in other systems incorporating lyotropic liquid crystals and the order of the liquid crystal is retained in the polymer structure.
The information included in this Background section of the specification, including any references cited herein and any description or discussion thereof, is included for technical reference purposes only and is not to be regarded as subject matter by which the scope of the present invention as claimed is to be bound.